1. Field of the Invention
The present invention relates to a switch circuit using field effect transistors (FETs) as switching devices, and more particularly to a switch circuit in which an electrical path is switched over between two paths by use of FETs.
2. Description of the Related Art
A single-pole double-throw (SPDT) switch having one input terminal and two output terminals has been used widely as a switch operating in a microwave band and in a millimeter-wave band. The SPDT switch is used, for example, for selectively connecting a radar antenna to a transmitter or a receiver.
In the SPDT switch a depletion-type metal-semiconductor field-effect transistor (MESFET) using compound semiconductor such as GaAs is inserted between the input terminal and each of the two output terminals, as shown in FIGS. 3 and 5 of an article "Broadband GaAs FET 2.times.1 Switches" by Y. Tajima et al., IEEE GaAs Symposium Digest, pp. 81-84, 1984. The depletion-type MESFET takes the "ON" state to show a low resistance when its gate electrode is at 0 volt and takes the "OFF" state to show a high resistance when its gate electrode is biased at a voltage below a threshold value.
The present inventor has newly found such a disadvantage of the SPDT switch when it is fabricated on a GaAs substrate that a difference in potentials appears between the input terminal and one of the output terminals on the side of the ON-state FET and causes an increase in the ON-resistance of that FET to increase the insertion loss between the input and output terminals and generate imbalance of insertion losses between the output terminals.